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Early Spring floods in 2007 had inundated the flat neighborhoods and farms around the eastern Indiana house of the Keesling family. Their home's basement had taken on some 30,000 gallons of water, and a gasoline pump had been set up to empty it. After the family went to bed, a crack in the pump's venting system caused carbon monoxide to pour into the home's heat ducts.

Cathy Keesling had closed all the windows in the house, save one on the first floor where Winnie, the gray-and-black-striped cat the family had rescued from a barn years before, was sleeping. When deadly gas filled the house, Cathy's teenage son, Michael, fell unconscious in the hallway. Cathy and her husband, Eric, were slowly sinking into unconsciousness as well. Winnie had been breathing the clear night air, so she was the only living creature in the house that could tell something was wrong. But rather than escaping through the open window, Winnie raced over to Cathy.

"Winnie was pulling my hair and yowling in my ear," Cathy recalls of her normally mellow cat's unusual behavior. "I would wake up and pass out again. Every time I passed out, Winnie would wake me up again."

Cathy managed to rouse herself and dial 911, but the gas knocked her out before she could tell the operator what was going on. The dispatcher sent out a state trooper and sheriff's deputies, who dragged the family onto the porch and into the fresh air. A firefighter found Winnie in a closet.

Everyone recovered after many hours in the hospital, where the dire nature of their situation became clear. "The deputy sheriff told me that if Winnie had waited five more minutes to get us up, we'd all be dead," Cathy Keesling says. "I'm so proud of her.

"I guess because we saved her life, she saved ours."

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In 1987 the National Institutes of Health (NIH) launched a workshop on the health benefits of owning pets: at that time there were a few scientific papers on the subject, including a study that proved pets boosted survival rates for coronary care unit patients. By last year, when the NIH held a workshop on how pets help people, new findings had proliferated, though in many cases they simply validated what people have known about pets for centuries. One study showed that dog ownership promotes regular exercise. Another found that being near a pet lowers its owner's blood pressure (an effect that family members, no matter how beloved, can't match).

More surprising were new data on the key role of chemistry in the relationship. When a person interacts with a pet, the central nervous system releases several hormones that cause feelings of pleasure—and one hormone in particular, oxytocin, appears to play a major role in reinforcing the bond. Produced by new mammalian mothers to encourage bonding with their offspring, oxytocin creates a sense of warmth, nurturing, and calm. In 2002 two South African researchers measured oxytocin levels not only in humans petting dogs but in the dogs themselves: the dogs experienced the same chemical releases and calming effects as did the humans. Researchers are still unclear about the exact role of these chemicals, though when two different species can produce feelings of peace, closeness, and contentment in each other, it's clearly an intriguing find.

Karen Grindler has seen firsthand the bond's healing effect—on both people and animals. Grindler runs the Cedar Creek Therapeutic Riding Center in Columbia, Missouri, where people with disabilities gain physical mobility and massive improvements in mood simply by riding horses. Grindler is full of tales about riders who have learned to walk again, or even just smile again, after a few weeks on horseback, whether it's loops around the paddock or a leisurely clop through the countryside.

Most dramatically, she tells the story of Kid, a 40-year-old horse (one of the oldest in the country) that has lived at Cedar Creek since 1997. "I tried to retire Kid in 2006," she says. "He was 37. Kid got depressed, really hung his head low." Soon enough, a young man named Jeremy Hardin arrived at the center. Lately, his cerebral palsy had begun to require that he take frequent rest breaks during a ride, which was difficult for Grindler's younger horses. "I pulled Kid out of pasture," Grindler recalls, "because I knew he wouldn't mind stopping."